Vehicle bodies are being manufactured from high strength, lightweight materials to reduce overall vehicle weight and improve fuel economy. Mild steel, the predominant structural material previously specified for vehicle structures, is ductile and does not tend to split easily when hydro-formed. Hydro-forming cylindrical tubes made of lightweight materials such as aluminum and high strength steel alloys into tubular parts is often limited by the maximum strain in a local area causing splits, generally near a corner, while the rest of the tube may have very low levels of strain. The thickness of the part and the weight of the tube is increased to reduce the formation of splits when lightweight materials are hydro-formed. Using thicker tubes increases the material cost of the parts. Increasing the thickness of the tubes also limits weight reduction and results in reduced fuel economy.
Hydro-formed parts often have polygonal or rectangular cross-sections that require corners to be formed in a cylindrical tubular blank. High stress areas are generally located near the corners and hydro-formed parts made of lightweight materials may split near the corners resulting in scrapping parts.
The method disclosed addresses the problems associated with hydro-forming structural parts from lightweight materials as summarized below.